The present invention relates to a process for the purification of acrylic acid or methacrylic acid by a combination of crystallization and distillation.
Acrylic acid is an important key chemical. Owing to its very reactive double bond and the acid function, it is particularly suitable as a monomer for the preparation of polymers, for example for adhesives, dispersions, surface coatings or superabsorbers.
It is generally known that acrylic acid can be prepared via acrolein by gas-phase oxidation of propene with molecular oxygen under heterogeneous catalysis over catalysts present in the solid state at from 200 to 400xc2x0 C. in two stages. Here, oxidic multicomponent catalysts, for example based on the oxides of the elements molybdenum, bismuth and iron (in the first stage) or molybdenum and vanadium (in the second stage) are used.
DE-A-196 06 877 describes a process for the preparation of acrylic acid or methacrylic acid, in which a product mixture containing the acrylic acid or methacrylic acid and prepared by catalytic gas-phase oxidation is absorbed with a high-boiling solvent, the laden solvent is separated into the solvent and a crude acid by distillation and the acrylic acid or methacrylic acid is obtained from the crude acid by crystallization.
EP-A-0 616 998 relates to a process for the purification of acrylic acid by means of fractional crystallization by a combination of dynamic and static crystallization in a plurality of stages, the residue of the dynamic crystallization being further purified by the static crystallization and the acrylic acid obtained being recycled to the dynamic crystallization.
EP-A-0 675 100 discloses a process for the preparation of xcex1,xcex2-unsaturated C3-C6-carboxylic acids and of acrylic acid, in which the acid is prepared by oxidative dehydrogenation of the corresponding saturated acid, the dehydrogenation product is crystallized from the melt, the mother liquor obtained is subjected to fractional distillation and the top product of the distillation is recycled to the dehydrogenation reactor and the bottom product of the distillation to the melt crystallization or the top product is recycled to the melt crystallization and the bottom product to the dehydrogenation reactor. In a further embodiment of EP-A-0 675 100, the product stream obtained in the oxidative dehydrogenation is first subjected to fractional distillation, the top product of the distillation is recycled to the dehydrogenation reactor and the bottom product of the distillation is fed to a melt crystallization and the mother liquor obtained in the melt crystallization is then recycled to the distillation.
It is an object of the present invention to provide a process in which acrylic acid or methacrylic acid is obtained in high purity and in high yield.
We have found that this object is achieved by the combined use of crystallization and distillation, the mother liquor formed in a first crystallization of the mixture containing acrylic acid or methacrylic acid being fed at least partly to a distillation, the resulting top product being subjected at least partly to further crystallization and the resulting crystals in turn being recycled to the first crystallization.
The present invention therefore relates to a process for the purification of acrylic acid or methacrylic acid by crystallization and distillation, which comprises the following steps:
(a) crystallization of a mixture containing the acrylic acid or methacrylic acid with formation of product crystals having a higher concentration of acrylic acid or methacrylic acid and of a mother liquor,
(b) distillation of at least part of the mother liquor from step (a) with formation of a distillation residue, which is at least partly removed, and of a top product,
(c) crystallization of at least part of the top product from step (b) with formation of a crystallization residue, which is removed, and of crystals and
(d) recycling of the crystals from step (c) to the crystallization (a).